Can someone please tell me what I’m doing wrong?
I’m creating an app that signs people into a facility. I have a log page that shows the three locations where people sign in. Each location is called a ‘checkpoint.’ Each checkpoint listed on the page uses a repeating group to display the people signed in at each location.
I have a CheckpointLog data type that collects all the sign in data from people. I have a Checkpoint data type that defines a checkpoint ID (unique ID), checkpoint name, and a link to the Sites data type (the facility checkpoints are associated with). Below are my Checkpoint Log and Checkpoints data types.

I’m trying to do a constraint on the repeating groups so that it only displays data from a particular checkpoint ID. I try searching for CheckpointLogs and doing something with first item or items related to checkpoints for the data source. In the constraint part, it’s not allowing me to do anything with Checkpoints. Below is what I see with constraints.
What do I need to be doing that I’m currently not doing?